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Abstract

The viability of the application of surface-enhanced Raman spectroscopy (SERS) to the semi-quantitative analysis of the triphenylmethane dye gentian violet was examined by using activated borohydride-reduced silver colloids. Raman and SERS spectra of aqueous solutions of gentian violet at different pH values were acquired for the first time and equally intense SERS signals were obtained at both acidic and alkaline pH values. Two maxima intensities observed in the pH profile revealed the presence of different ionization states of the dye. The pH conditions for SERS were optimized over the pH range 1 to 12 and the biggest enhancement for SERS of this charged dye was found to be at pH 2.0; thus, this condition was used for semi-quantitative analysis. A good linear correlation was observed for the dependence of the signal intensities of the SERS bands at 1620 cm<sup>–1</sup>(<i>R</i> = 0.999) and 1370 cm<sup>–1</sup> (<i>R</i> = 0.952) on dye concentration over the range 10<sup>–6</sup> to 10<sup>–4</sup> mol/L, using laser excitation at 514.5 nm. At concentrations of dye above 10<sup>–2</sup> mol/L, the concentration dependence of the SERS signals is nonlinear. This is explained as due to the precipitation of metallic silver as well as due to saturation caused by complete coverage of the SERS substrate. A series of intensities of the band at 1620 cm<sup>–1</sup> measured from dye molecules proved that the single-molecule limit of gentian violet is attained at the concentration of 10<sup>–9</sup> mol/L.